1. Field of the Invention
The present invention relates to a power module used for controlling a motor of an industrial or consumer apparatus.
2. Description of the Background Art
A power switching semiconductor device such as an IGBT (Insulated Gate Bipolar Transistor) or a power MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) adopts the following method to detect an overcurrent. The power switching semiconductor device is configured of a main element through which a main current flows and a current sense element through which a part of the main current flows; a sense current outputted from an output terminal (sense emitter) of the current sense element is converted, by a resistor (current sensing resistor), into a voltage to serve as a detection voltage; and the detection voltage is compared with a predetermined reference voltage to determine whether the detection voltage is normal or abnormal (overcurrent level).
Here, the current sense element is structured of a collector (drain) which is shared by the main element, and an emitter (source) having an area which is arranged to have a predetermined area ratio to an area of an emitter (source) of the main element so that a sense current at a predetermined division ratio to the main current flows therethrough.
For example, in the case where an emitter area ratio of the current sense element with respect to the main element is 1/10000, a current which is 1/10000 of the current of the main element flows through the current sense element. This makes it possible to detect a current by a resistor having a relatively smaller resistance.
Here, when the current sensing resistor is connected to the current sense element, a difference between voltages of gates of the main element and the current sense element is caused, which results in a change of a current division ratio. Since this division ratio changes largely when a resistance of the current sensing resistor is large, it is necessary to use a smaller resistance for sensing.
However, when a smaller resistance is used for sensing, a threshold voltage (reference voltage) for determining an overcurrent becomes smaller. This causes a factor for a malfunction (false detection).
Japanese Patent Application Laid-Open No. 10-322185 (1998) discloses, in FIG. 1, a configuration in which a sense current is not directly sensed by a resistor, but is received by a current mirror circuit configured of an N-channel MOS transistor, and a mirror current (current I4) obtained by the current mirror circuit is converted into a voltage by a current sensing resistor (resistor R1) connected to a power source (voltage V3) of the current mirror circuit to thereby form a detection voltage (voltage V1).
According to this configuration, the detection voltage V1 is expressed as V1=V3−(I4×R1). Since the detection voltage V1 depends on the voltage V3 of the power source, the detection voltage V1 varies according to a change of the voltage V3, and this may lead to a drop in accuracy of current sensing.
A similar problem is also caused in a technique disclosed in Japanese Patent Application Laid-Open No. 1-193909 (1989) (FIGS. 1 and 2) in which a current mirror circuit for receiving a sense current and generating a mirror current is combined with a current mirror circuit for generating a reference current as a mirror current, and the presence or absence of an overcurrent is determined according to a magnitude relation between the mirror current of the sense current and the reference current. Also, in this case, when a source voltage of the current mirror circuit changes, the mirror current changes, and a drop in accuracy of current sensing may be caused.
As described above, according to a conventional configuration of a power switching semiconductor device for sensing an overcurrent, in the case where a sense current is sensed by a resistor, a variation thereof becomes large when a current sensing resistance is large, and a false detection tends to be caused when the current sensing resistance is small. Also, in the case where the sense current is sensed by using a current mirror circuit, a drop in accuracy of current sensing may be caused with fluctuations in power source.